This invention relates generally to zoom lenses and more specifically to calibration of the zoom lens position.
Zoom lenses are employed by numerous types of imaging devices such as still and video cameras, etc. Some types of zoom lens assemblies include imprecise and/or low-resolution components such as drive systems or zoom position sensors which are employed because of their other advantages, such as low cost. Because of these imprecise and/or low-resolution components, a zoom lens assembly with a discrete number of zoom positions (or focal lengths) may come to rest at an offset from the desired zoom positions. Further, the direction in which a zoom position is approached by the zoom lens affects the direction of the offset from the desired zoom position. For example, if a zoom lens assembly includes focal lengths of 10 mm, 12 mm, and 14 mm among the discrete zoom positions measured by its discrete position sensor, and the zoom lens assembly is zoomed to the 12 mm position, the actual resulting offset from 12 mm differs depending upon the direction from which the zoom lens approaches 12 mm. If the zoom lens zooms from 10 mm to 12 mm, it may overshoot the 12 mm position by an offset, coming to rest at an actual focal length of 12.3 mm, for example. If the zoom lens zooms from 14 mm to 12 mm, it may overshoot the 12 mm position by an offset in the other direction, coming to rest at an actual focal length of 11.8 mm.
Thus, a zoom lens with this type of discrete position sensor can typically be positioned only at small offsets from the predetermined zoom positions, with the direction of offset dependent upon the direction the zoom lens was last traveling.
An embodiment of the invention enables a zoom lens with discrete focal length position sensors to be positioned at arbitrary locations between the predetermined zoom positions. This is accomplished by calibrating the movement of the zoom lens to determine the time required to move past the predetermined zoom positions to reach a desired location or focal length. Thus, the zoom lens can be adjusted to many focal lengths based on the discrete focal length position sensors and the duration of movement. This allows the zoom lens to reach a greater number of focal lengths than the few zoom positions provided by the discrete sensors. Using this zoom lens calibration, the zoom lens can also be accurately positioned without overshoot offsets regardless of the last direction of travel. This eliminates the need to perform a zoom direction reversal at the end of certain zoom operations.